A time-of-flight mass spectrometer is a widely used tool of analytical chemistry, characterized by high speed analysis of wide mass ranges. It has been recognized that multi-reflecting time-of-flight mass spectrometers (MR-TOF-MS) provide a substantial increase in resolving power due by reflecting the ions multiple times within the flight region so as to extend the flight path of the ions. Such an extension of the ion flight paths requires folding ion paths either by reflecting ions between ion mirrors or by deflecting ions in sector fields. MR-TOF-MS instruments that use ion mirrors provide an important advantage of larger energy and spatial acceptance due to high-order time-per-energy and time-per-spatial spread ion focusing.
FIG. 1 illustrates a known MR-TOF-MS instrument, e.g. as described in SU 1725289. The instrument comprises two two-dimensional ion mirrors 12 extended along a drift dimension (Z-direction) for reflecting ions, an orthogonal accelerator 13 for injecting ions into the device, and a detector 14 for detecting the ions. For clarity, throughout this entire text the planar MR-TOF-MS is described in the standard Cartesian coordinate system. That is, the X-axis corresponds to the direction of time-of-flight, i.e. the direction of ion reflections between the ion mirrors, the Z-axis corresponds to the drift direction of the ions, and the vertical Y-axis is orthogonal to both the X and Z axes.
Referring to FIG. 1, in use, ions are accelerated by accelerator 13 towards one of the ions mirrors 12 at an inclination angle α to the X-axis. The ions therefore have a velocity in the X-direction and also a drift velocity in the Z direction. The ions enter into a first of the ion mirrors 12 and are reflected back towards a second of the ion mirrors 12. The ions then enter the second mirror 12 and are reflected back to the first ion mirror 12. The first ion mirror then reflects the ions back to the second ion mirror 12. This continues and the ions are continually reflected between the two ion mirrors 12 as they drift along the device in the Z-direction until the ions impact upon detector 14. The ions therefore follow a substantially sinusoidal or zigzag (jigsaw) mean trajectory within the X-Z plane. The ions advance along the Z-direction for each mirror reflection with an incremental distance of ZR=C*sin α, where C is the flight path per one ion mirror reflection. However, no ion focusing is provided in the drift Z-direction and so the ion packets diverge in the drift Z-direction. This drawback limits the duty cycle of the spectrometer, for example, to less than 0.5% at a mass resolving power of 100,000.
It is known, e.g. from WO 2005/001878, to provide a set of periodic lenses within the field-free region between the ion mirrors so as to prevent the ion beam diverging significantly in the Z-direction, thereby overcoming the above described problem. However, it has been discovered that the ion optical elements of the instrument, including the periodic lenses, limit the practical applications of the analyser.
It is desired to provide an improved spectrometer and an improved method of spectrometry.